This invention relates to a prosthesis shaft for the implantation of a prosthesis, especially a part of an artificial hip, knee or finger joint by means of a bone cement.
With conventional prostheses, the shaft which is to be anchored within the bone has a substantially smooth surface of conical configuration. FIGS. 1 and 2 illustrate schematically by way of a hip prosthesis the retaining forces which occur on the surface of the shaft 10 when a force P acts on the articular head 11. These retaining forces can be resolved into forces of pressure (+) and tension (-) resulting from the bonding moment of the force P multiplied by the distance e of the direction of the force from the shaft axis, and transverse or shearing forces (.uparw..dwnarw.) resulting from the axial force P and acting parallel to the shaft surface. The actual load on the bone cement anchoring the shaft results from a superposition of these compressive, tensile and shearing forces.
It is known from EP-A-No. 0,212,084 to anchor a prosthesis shaft of overall conical design by means of bone cement, the shaft surface being additionally provided with cylindrical recesses.
Known bone cements are able to transmit forces of pressure and tension equally well and can also accommodate shearing forces relatively easily. But it is a significant drawback of the known bone cements that they attack the bone with the result that the life of the anchoring bond is limited and that a later re-anchoring is hardly possible because the bone is then partly destroyed.
For this reason, it has frequently been attempted to anchor prostheses in the bones without any cement. DE-A-No. 2,461,339 discloses such a prosthesis the shaft of which is substantially formed of a flat metal plate having its narrow sides provided with rounded steps within the bone. These steps are configured so that each one has an edge face directed perpendicularly to the trajectorially oriented spongiosa structure for direct transmission of the local forces of pressure and tension. The thus produced approximately sawtooth-like design of the edge faces is also intended to increase the overall area of engagement between the shaft and the bony tissue, thereby reducing the pressures acting on that area. However, the cement-free bonding attempted with such a shaft requires good growth of the bony tissue so that the sawtooth-like shaft faces are securely embedded in the required way. Even if this requirement is initially met, there will always be a risk of the cement-free anchoring to loosen in the course of time.
DE-A-No. 3,445,738 further discloses a generally hollow-cylindrical bone peg having an internal thread for receiving a bone screw and an external surface which is formed by a plurality of large spherical surfaces partially penetrating each other and being provided with axial and/or transverse slots and an additional relief, notably in the form of small spheres. This shape is intended to achieve an intimate bond with the bony tissue retained so that the cement-free anchored prosthetical part may be embedded by natural growth. Thus, a sufficient growth of the bone is again a prerequisite, and there is still the risk of the prosthetical part loosening later on.
Recently, it has been considered to use glass ionomers as bone cements, since glass ionomers are bio-inert and will thus, contrary to the conventional cements, not affect the bony tissue. Though glass ionomer-type bone cements exhibit very good compressive strength, they have little tensile strength and exhibit significant brittle fracture behaviour.